Class template to_python_indirect observers

PyObject* operator()(T x) const;

Requires:x refers to an object (if it
is a pointer type, it is non-null). convertible() ==
true.

Effects: Creates an appropriately-typed Boost.Python
extension class instance, uses MakeHolder to create
an instance_holder from x, installs
the instance_holder in the new extension class
instance, and returns a pointer to it.